1. Field of the Invention
The present invention relates to a displacement measuring device with an interference grating, which allows a light beam from a coherent light source to be incident on a scale in a plurality of directions and respective diffracted light beams to interfere with each other, thereby providing a detection signal. The device is suitable for use, among other things, in a linear encoder for measuring the relative displacement between a sensor portion and a scale, a measuring apparatus such as a linear gauge incorporating the linear encoder or a shape measuring apparatus, a measuring device such as for measuring inner and outer diameters, and apparatus for positioning or controlling the speed of a moving stage of a machining tool or an inspection machine.
2. Description of the Related Art
Optical encoders have become widely available which employ a scale having optical calibration markings formed at a constant pitch to generate cyclic detection signals. Furthermore, as one of the optical encoders of this type having an improved high resolution, a device with an interference grating for detecting displacement has been suggested, in which the scale is provided with calibration markings at a fine pitch using the holography technique so as to use the calibration markings as a diffraction grating to positively cause diffraction, thereby providing a detection signal.
FIG. 1 illustrates a device with an interference grating for detecting displacement, which the present applicant has suggested in Japanese Patent Laid-Open Publication No. Hei 1-185415. The displacement measuring device includes a scale 10 formed of a diffraction grating having a pitch p of the same order as a light source wavelength λ, for example, 1 μm or less. The device also includes a sensor portion 20 that has a coherent light source (also hereinafter simply referred to as a light source) 32, such as a laser diode (LD), for illuminating the diffraction grating with a laser beam 14 of a wavelength λ, a collimator lens 34, light-receiving elements 22A, 22B, and 22C for optoelectronic conversion of a combined wave of a plurality of light beams yielded by the diffraction grating, polarizers 28B, 28C, and a quarter-wave plate 30. The device is configured to generate a detection signal that varies periodically depending on the relative displacement between the scale 10 and the sensor portion 20. In this measuring device, the sensor portion 20 includes a half mirror 40 for halving the laser beam 14 from the light source 32, and a pair of mirrors 42A and 42B for allowing the halved laser beams to be incident symmetrically upon the same diffraction point 10A on the diffraction grating at the same angle of incidence θ. The measuring device is further set to have an angle of incidence θ and an angle of diffraction φ(φ<θ) that differ from each other to such an extent that a zeroth-order light beam of one of the halved incident light beams a and b and a first-order (diffracted) light beam of the other light beam can be separated from each other on the diffraction grating. The separated first-order light beams are reflected on a pair of mirrors 44A and 44B to be polarized orthogonally to each other by polarizers 46A and 46B, and their respective combined wave is then allowed to be incident on the light-receiving elements 22A, 22B, and 22C through half mirrors 48 and 50.
In this displacement measuring device, a light beam is incident and diffracted on the scale 10 at the angle of incidence θ and the angle of diffraction φ which are different from each other. Additionally, since the device is adapted to measure the relative displacement between the scale 10 and the other portion (the sensor portion 20), the device is attached to other apparatus, so that the scale 10 and the sensor portion 20 are mounted on separate members to allow either one of them to be displaced.
However, with the arrangement of this optical system, suppose that a pitch angle (the positional relationship of rotational directions on the drawing of FIG. 2 showing the main portion of the optical system) between the sensor portion 20 and the scale 10 is deviated from the proper position. In this case, the angle of transmission φ from the scale 10 takes on different values for the right and left optical paths. Thus, as shown in FIG. 3, the measured values of the output signal level against the pitch angle with the pitch angle being varied teach that the contrast is reduced due to interference, causing the output signal level to be degraded. This raised a problem that a sufficient performance could not be made available.
Accordingly, to make full use of the function of a device with an interference grating for measuring displacement which has the optical system shown in FIG. 1, it is necessary to adjust the pitch angle, when attached to an apparatus, so as to provide an output signal of the maximum level. Furthermore, an additional adjustment also needs to be made for another direction, thereby requiring adjustments for two directions. This raised another problem that it took time to attach the device to an apparatus.
In Japanese Patent Laid-Open Publication No. Hei. 1-185415, a modified example is also described in which the angle of incidence θ is generally equal to the angle of diffraction φ as shown in FIG. 4. However, since the zeroth-order light beam and the first-order light beam were inseparably mixed up with each other, good signals could not be obtained.